The present invention is directed to liquid chromatography and is particularly concerned with an arrangement for critical angle refractive index detectors in production chromatography systems.
In liquid chromatography a liquid sample is passed by a flowing stream of liquid solvent (the mobile phase) through a column packed with particulate matter (stationary phase). While passing through the column the various components in the sample separate from one another by adsorbing and desorbing from the stationary phase at different rates such that these individual components elute from the column at different times. The separating components then flow through a detector which responds to each component both qualitatively and quantitatively thereby providing information to the user about the constituents of the sample. The particulate matter in the chromatography column is generally referred to as the chromatography media and the resolution of the sample into its individual components by the chromatography media is a primary measure of the chromatography separation performance.
Critical angle refractive index detectors are traditionally used by heavy industries as industrially rugged monitors in large scale process pipelines. The detector is used to monitor the composition of the mixture flowing through the pipe. The traditional applications are concerned with utilizing the detector to monitor a blended stream, whose refractive index is used as a key parameter to verify the composition of the mixture being blended, e.g., syrup formulations for soda, petroleum oils, etc.
The flow rates typical in these industrial processes range between 50 to 400 liters/minute (l/min). The detector sensitivity is usually adjusted to monitor a highly specific range, e.g., a mixture whose refractive index is 1.003, will have a range set of 1.000 to 1.006. The range controls are usually internally set and adjusted only for calibrations. In contrast, chromatography applications would call for flow rates as low as 100 ml/min, and ranges of detection which change on a daily basis or even more frequently. Range controls for zero and sensitivity (span) would need to be readily accessible to the end user. The goal of the chromatography application is not to monitor a blended stream for consistent composition but to detect compounds which have been separated and purified as they elute the chromatography column. The chromatographer is not concerned with the true or real value of the refractive index of the mobile phase and eluting peaks; the concerns are in distinguishing the presence of eluted peaks in the mobile phase as they occur, i.e., the detector's response to the temporary presence of eluting compounds.
The critical angle detector utilizes the principles of refractive index in combination with a prism, to measure light returned as a function of the critical angle which, in turn, is a function of the refractive index of a liquid flowing past the prism. The only surface in contact with the liquid is that of the solid prism. An LED light source emits light through the prism, some of which is lost into the liquid, while the remaining amount is refracted and returns through the prism and is measured by a photodiode generating a signal whose response is proportional to the liquid's refractive index properties at the prism surface. The result is a rugged detector which can withstand high flow rates and large back-pressures (as high as 4000 psi).
Traditional refractive index detectors used in chromatography are known as differential refractive index detectors. The flow cell is usually constructed of delicate glued sections of quartz, resulting in a fragile cell. The cell is comparted with a sample side and reference side. Maximum flow and pressure rating tend to be in the 100 ml/min, 100 psi range. The detector, however, can be made quite sensitive to minute concentrations of sample, which in analytical chromatography is a key criterion. In preparative applications, such high sensitivity is not required. In the past, preparative chromatography equipment would use differential refractive index detectors and would, due to flow rate limitations of the flow cell, split a small sample stream through the detector's flow cell ("split stream detection"). The approach has been cumbersome with maintenance problems and performance issues as to whether the split stream sample is truly representative of the main stream composition.